A public lecture titled The Mystery of Dark Energy: Why Is the Universe Still Accelerating? was held on 15 May at the Municipal Library in Prague. The talk was delivered by Professor Bharat Ratra of Kansas State University, recipient of the 2025 Julius Edgar Lilienfeld Prize of the American Physical Society. The event opened with remarks from Luke Meinzen, cultural attaché at the US Embassy, who together with Dr. Ignacy Wolak-Sawicki welcomed Professor Ratra to the venue.

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In his lecture, Professor Ratra outlined the modern standard model of cosmology, beginning with rapid inflation after the Big Bang and tracing the development of structure in the universe. The talk addressed unresolved questions, including the mismatch in current measurements of the expansion rate, and noted that future telescopes such as the James Webb Space Telescope may provide further data. The presentation also covered the scales of the universe and the limits of Earth‑based intuition, explaining the roles of dark matter and dark energy in shaping present‑day cosmological models.

We usually imagine dark matter as a swarm of heavy, invisible particles. But are there other possibilities? What if dark matter isn't a storm of sand, but rather a vast, invisible ocean that hums?  

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The invisible web of dark matter makes up more than a quarter of the cosmos and indirectly affects all other forms of matter and energy through its gravitational pull. Astronomers derive its presence by observing stars, galaxies, clusters of galaxies, and a relict ancient light from a time when the Universe was only about 380 000 years old.  

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Despite decades of searching using ultra-precise detectors built with cutting-edge technology, we have never directly detected these dark matter particles in our laboratories. However, recent technological advances enable us to push the detection sensitivity to new levels; we are currently living through a golden age for dark matter science.  

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A Universe That Hums

The standard model of cosmology and dark matter struggles to explain certain fine-scale properties, such as the inner structure of dwarf galaxies or the abundance of satellite galaxies orbiting the Milky Way. Ultra-light dark matter, also called wave dark matter, is an entirely different proposition.  In this concept, dark matter consists of particles so incredibly light that they behave more like waves moving through the cosmos rather than individual billiard balls.  

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Because ultra-light dark matter acts like a wave, we don't catch it by smashing atoms together. Instead, we "hear" it through careful listening. "Think of it as a background hum," explains Federico Urban of the FORTE team. " Latest advancements in the field of quantum sensing have led to a significant progression in laboratory searches for these signals."

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At FORTE, we try to listen to the hum of the wave-like Universe through the distinctive approach of combining laboratory experiments with astrophysical observations. The FORTE team is tuning in across all frequencies to explore the unknown properties of dark matter, listening carefully to the whispers of the dark Universe.

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Terrestrial Quantum Sensors:

For our research we use data from some of the most sensitive instruments ever built. We also design new experiments that are uniquely geared to catch the dark matter waves and, if we are successful, determine its properties such as its mass and how it interacts with light and ordinary matter.

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Gravitating Seismographs: Gravitational wave detectors use lasers bouncing between mirrors to measure ripples in spacetime itself. If a dark matter wave washes through, it will set the mirrors trembling, creating shifting interference patterns that we can observe.

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Quantum Echo Chambers: Experiments such as QUAX or MADMAX act like high-precision bells. We wait for the "wind" of dark matter to strike them, causing the resonant cavities to ring at a specific microwave frequency.

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Atomic Rulers: Atom interferometers and atomic clock networks enable physicists to measure time and distance with extraordinary precision. If a wave of dark matter passes through the laboratory, it could subtly disturb the ticking of these clocks or the position of the atom clouds – and we can detect these tiny disturbances.

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Levitated Sensors: Suspending tiny particles in vacuum using lasers or magnetic fields and monitoring their vibrations, enables physicists to detect waves of dark matter passing through.

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The Cosmic Laboratory:  

Our search doesn't stop at the laboratory door. The Universe itself provides us with natural detectors on a truly colossal scale. FORTE researchers analyse astrophysical data to find the fingerprints of ultra-light dark matter in the sky:

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Cosmic Lighthouses: Astronomers monitor pulsars – dead stars that spin hundreds of times per second with an astonishingly regular frequency, beaming radio waves to the Earth like a lighthouse. They are amongst the most precise clocks in the entire Universe. If the space between Earth and a pulsar is filled with travelling dark matter waves, the arrival time of those flashes will flicker. We analyse data from pulsar timing arrays, a collection of radio telescopes observing pulsars, to hunt for these minute variations.

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Black Hole Superradiance: Rotating black holes can “grow” a cloud of dark matter around them, similar to an electron cloud surrounding an atomic nucleus. This process, called superradiance, extracts rotational energy from the black hole, slowing its spin and emitting continuous gravitational waves that we can detect on the Earth; these gravitational waves then teach us about the dark matter clouds that triggered them.

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Warped Galaxies: By using observations related to how star clusters, the dynamics of satellite haloes, and how stellar streams move around larger galaxies, we map the texture, called "granularity", of dark matter haloes to determine whether their motion matches the wave-like patterns predicted by our models.

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